Vol 53, No 1 (2017)

Improvement of reinforcement schemes and production processes using different types of carbon matrices and reinforcing fibers makes it possible to change the properties of carbon-carbon composites in a wide range depending on the operating conditions. One of the most promising ways to improve oxidation resistance of the composites is a bulk liquid silicon infiltration, which makes it possible to use them in optical and friction systems, as a ballistic protection of vehicles and aircraft, as cutting and grinding tools, and as a thermal protection of spacecrafts.

The impurity composition of ²⁸SiH₄, ²⁹SiH₄, and ³⁰SiH₄ silanes and ⁷²GeH₄, ⁷³GeH₄, ⁷⁴GeH₄, and ⁷⁶GeH₄ germanes isotopically enriched to above 99.9 at % has been studied by gas chromatography/mass spectrometry using capillary adsorption columns. Impurities have been identified by comparing their mass spectra with NIST data and information available in the literature, and by inferring their structure from fragment ions and retention times. We have identified 53 impurity substances in silanes and 42 in germanes: permanent gases; saturated, unsaturated, halogen-containing, and aromatic C₁–C₉ hydrocarbons; their homologues; alkyl derivatives of silane and germane; chlorogermane; siloxanes; fluorosiloxanes; sulfur compounds; and dioxane. The silicon- and germanium-containing impurities have been shown to be isotopically enriched, as the major component. The detection limits of the impurities are 5 × 10^–8 to 3 × 10^–5 vol %, comparing well with the best results in the literature.

Nanostructured ZnS:Mn films have been grown and their structure, optical properties, and photoluminescence have been studied. The nanostructured ZnS:Mn films have been grown on silicon and glass substrates via hydrochemical deposition from solution. The crystal structure and microstructure of the films have been studied by X-ray diffraction and atomic force microscopy. The band gap of the nanostructured ZnS:Mn films has been determined. The intensity of their photoluminescence bands has been shown to increase with decreasing nanoparticle size.

We have studied the thermoelectric properties of p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.4Se_0.6 solid solutions prepared by vacuum hot pressing of mixtures of powders differing in particle size composition. The powders were prepared by mechanically grinding ingots and by ultrarapid melt cooling (melt spinning). Fracture surfaces of samples were examined by scanning electron microscopy. The samples consisted of large, layered particles of the major component (up to hundreds of microns in size) and small flakes (a few to tens of microns in size) of the component prepared by melt spinning. The microstructure of the materials was examined under an optical microscope. On grain boundaries in the p-type materials, we observed telluriumbased eutectic precipitates in the form of a white phase. In some of the n-type samples, the Bi₂Te_2.4Se_0.6 solid solution was found to undergo ordering, resulting in the formation of the ternary compound Bi₂Te₂Se. The Seebeck coefficient, electrical conductivity, and thermal conductivity of the materials were measured in the temperature range 100–700 K. The addition of 40 wt % powder prepared by melt spinning to hot-pressed p-type samples was shown to have no effect on their thermoelectric figure of merit (ZT)_max, which was 1.0 at 350 K. On the addition of 20 wt % powder prepared by melt spinning, we obtained (ZT)_max = 1.1 at 550 K in a hot-pressed n-type sample.

It has been shown in the process of optimization of approaches to effective control over the formation of functional nanofilms on III–V semiconductors that the surface modification of InP with magnetronsputtered MnO₂ nanolayers (25 nm thick) results in an oxygen transfer mechanism of the thermal oxidation of the semiconductor. The advantages of this approach are a higher rate of the increase in film thickness in comparison with stimulator-free oxidation, rapid chemical binding of indium, blocking of indium diffusion into the film, and accelerated phosphate formation. Changes in the composition of the films in comparison with those produced by stimulator-free oxidation lead to a considerable improvement in their surface quality (the roughness height does not exceed 20 nm and the average grain size is 55 nm).

Low-temperature (<500°C) stages in the synthesis of gamma-lithium monoaluminate through heat treatment of a mechanically activated mixture of aluminum hydroxide and lithium carbonate have been studied by thermogravimetry and differential scanning calorimetry in combination with mass spectrometry. The results demonstrate that heating the mixture to above 80–100°C is accompanied by the release of not only water (resulting from the decomposition of X-ray amorphous aluminum hydroxide) but also carbon dioxide. The carbon dioxide originates from the reaction of the lithium carbonate with the products of X-ray amorphous aluminum hydroxide thermolysis.

The low-temperature heat capacity of Dy₂O₃ · 2ZrO₂ and Ho₂O₃ · 2ZrO₂ has been determined by adiabatic calorimetry in the temperature range 10–340 K. The results have been used to calculate the entropy, enthalpy increment, and reduced Gibbs energy of the zirconates without taking into account their low-temperature magnetic transformations.

We have studied the effect of synthesis conditions on the phase composition, grain size and morphology, degree of superstructural ordering, and magnetic properties of the Sr₂FeMoO_6–δ double perovskite. The results demonstrate that its magnetic state, dependent on the nonuniformity of grain morphology and the degree of superstructural ordering of the iron and molybdenum cations, correlates with the initial solution pH. Analysis of the sequence of phase transformations during strontium ferromolybdate crystallization in the citrate gel process from a pH 4 starting solution allowed us to propose combined conditions that ensure the preparation of single-phase Sr₂FeMoO_6–δ powder with an average grain size in the range 50–120 nm and the highest degree of superstructural ordering of the iron and molybdenum cations: P = 88%.

We have studied the electrical properties of ceramic Nb_2(1–y)Ta_2yO₅ samples, examined the dispersion of the real part of their dielectric permittivity ε′ at room temperature, and assessed their lattice dielectric permittivity ε′_∞ The temperature dependences ε′(T) have been obtained at different frequencies, and the dielectric loss tan δ has been evaluated as a function of measuring electric field frequency and temperature. The direct-current conductivity (σ_dc) of the solid solutions has been determined at room temperature. Using impedance spectroscopy data, we have obtained temperature dependences of σ_dc for the Nb_2(1–y)Ta_2yO₅ ceramics and evaluated the activation enthalpy for charge transport in these materials in different temperature ranges.

Using an arc physical vapor deposition process, we have produced nanostructured Mo–Si–Al coatings with a uniform distribution of equiaxed grains 8–12 nm in size and Mo–Si–Al–N coatings with a multilayer structure and a modulation period from 22 to 25 nm. The former coatings consist of MoSi₂ and Mo and the latter consist of Mo2N and amorphous Si₃N₄ and AlN. The hardness of the Mo–Si–Al–N and Mo–Si–Al coatings is 41 and 18 GPa, respectively; they are similar in resistance to elastic deformation; and the Mo–Si–Al–N coating has a considerably higher resistance to plastic deformation. The coatings have roughly identical coefficients of friction (~0.67–0.69 at 20°C and ~0.52–0.56 at 550°C), but the wear resistance of the Mo–Si–Al–N coating is higher by three and two orders of magnitude at 20 and 550°C, respectively. The coatings of the two systems exhibit good adhesion to the substrate and cohesive fracture. Partial wear of the Mo–Si–Al and Mo–Si–Al–N coatings in the course of scratch testing occurs at indentation loads of 80 and 63 N, respectively.